CLICK TRAIN ENCODING IN PRIMARY AUDITORY-CORTEX OF THE AWAKE MONKEY -EVIDENCE FOR 2 MECHANISMS SUBSERVING PITCH PERCEPTION

Multiunit activity (MUA) and current source density (CSD) patterns evo ked by click trains are examined in primary auditory cortex (A1) of th ree awake monkeys. Temporal and spectral features of click trains are differentially encoded in A1. Encoding of temporal features occurs at rates of 100-200 Hz through phase-locked activity in the MUA and CSD, is independent of pulse polarity pattern, and occurs in high best freq uency (BF) regions of A1. The upper limit of ensemble-wide phase-locki ng is about 400 Hz in the input to A1, as manifested in the cortical m iddle laminae CSD and MUA of thalamocortical fibers. In contrast, enco ding of spectral features occurs in low BF regions, and resolves both the f(0) and harmonics of the stimuli through local maxima of activity determined by the tonotopic organization of the recording sites. High -pass filtered click trains decrease spectral encoding in low BF regio ns without modifying phase-locked responses in high BF regions. These physiological responses parallel features of human pitch perception fo r click trains, and support the existence of two distinct physiologica l mechanisms involved in pitch perception: the first using resolved ha rmonic components and the second utilizing unresolved harmonics that i s based on encoding stimulus waveform periodicity. (C) 1998 Acoustical Society of America. [S0001-4966(98)01611-7]